(a) Field of the Invention
The present invention relates to a process for disposing waste gas containing sulfur oxides discharged from a coal thermal power station and specifically relates to a process for disposing waste gas that collects sulfuric acid mist contained in the waste gas.
(b) Description of the Related Art
Harmful substances such as sulfur compounds prepared from sulfur contained in fuel and nitrogen oxides obtained by oxidizing nitrogen in a combustion chamber at high temperature and high pressure state are contained in waste gas discharged from a boiler of a coal thermal power station, in addition to carbon dioxide being a main component. Consequently, the waste gas is discharged in atmosphere through the collection treatment of denitrogenation and desulfurization in series so that the waste gas discharged does not affect peripheral environment. In a general waste gas disposal apparatus, a denitration apparatus, dry type electrostatic precipitator, a wet desulfurization apparatus and wet type electrostatic precipitator are arranged in order from a boiler side and connected in series through piping.
The denitration apparatus is an apparatus decomposing nitrogen oxides (NOx) in the waste gas to harmless nitrogen (N2) and water vapor (H2O) by the action of a catalyst using ammonia (NH3).
The dry type electrostatic precipitator is an apparatus providing electric charge to particles such as dusts floating in air by corona discharge emitted from a discharge electrode and collecting the particles to a dust-collecting electrode by Coulomb force.
The wet desulfurization apparatus is an apparatus for carrying out the desulfurization treatment of the waste gas by spraying slurry comprising slurry materials such as lime hydrate and magnesium hydroxide for the waste gas to carry out gas-liquid contact, absorbing sulfur dioxide and the like in the slurry through chemical change with the slurry materials and discharging them out of the system.
The wet type electrostatic precipitator is an apparatus that adds a function of jetting water on the dust-collecting electrode of the dry type electrostatic precipitator with a spray and the like and can always discharge mists and dusts out of the system.
The sulfur compounds contained in the waste gas are mainly sulfur dioxide (SO2), but one portion is changed to sulfur trioxide (SO3) by combustion in a boiler and the catalytic oxidation of the denitration apparatus and further, sulfur trioxide is converted to sulfuric acid if it is reacted with water in the wet desulfurization apparatus. When the concentration of sulfur trioxide is several tens ppm, it is gaseous if temperature is one hundred several tens degree or more but when the gas temperature is the dew point of acid or less (for example, when the concentration of SO3 is 1 to 100 ppm, the dew point of sulfuric acid is 120° C. to 150° C.), it is condensed to be sulfuric acid mist. Since the sulfuric acid mists are corrosive, the temperature of the waste gas is controlled at higher temperature than the dew point of acid, for example, at about 170° C. or more by an air heater and the generation of the sulfuric acid mist is suppressed at a frontal stage of the wet desulfurization apparatus.
By the way, since the wet desulfurization apparatus has the highest desulfurization performance nearby the dew point of water, a large quantity of circulation water is sprayed in the apparatus. Accordingly, in the wet desulfurization apparatus, the temperature of the waste gas is abruptly lowered from about 170° C. to about 50° C. to 60° C. being the dew point of water. At this time, sulfuric acid in the waste gas is converted to mists at the lowering of temperature in the wet desulfurization apparatus. Since the particle diameters of the sulfuric acid mists by such abrupt cooling are small, collision probability with the sprayed slurry is low and it is difficult to remove it in the wet desulfurization apparatus. In this case, the sulfuric acid mists flow to the wet type electrostatic precipitator at a posterior stage. Further, when the waste gas is abruptly cooled, mists with a small particle diameter are generated, but since the finer the particles are, the higher the vapor pressure of particle surface is, coagulation effect using this as coagulation nuclei is not obtained and the diameters of mists cannot be enlarged.
On the other hand, the particle diameters of mists being collection objectives affect greatly collection performance in the wet type electrostatic precipitator and when the particle diameters are small, migration speed is little; therefore collection efficiency is greatly lowered. In this case, it is necessary to increase electric charge time for the sulfuric acid mists with small particle diameters in order to collect the sulfuric acid mists that passed the wet desulfurization apparatus, and consequently, the apparatus capacity of the wet type electrostatic precipitator must be enlarged; therefore the apparatus is jumboized. In particular, when the particle diameters of the sulfuric acid mists are submicron order with 1 μm or less, it is difficult to charge and there is fear that the sulfuric acid mists pass the wet type electrostatic precipitator and are discharged out of the system. Consequently, it is necessary to enlarge the particle diameters of the sulfuric acid mists to micron order in order not to discharge the sulfuric acid mists out of the system without loading the wet type electrostatic precipitator.
As means for solving the problem, Japanese Unexamined Patent Publication No. 2002-45643 discloses a method of spraying liquid such as water to waste gas introduced into the wet desulfurization apparatus, cooling the temperature of the waste gas to 120° C. to 150° C. and keeping the cooling temperature for 0.5 sec or more. According to the method, the average particle diameter of the sulfuric acid mists in the waste gas introduced into the wet desulfurization apparatus can be enlarged to micron order. Consequently, the average particle diameter of the sulfuric acid mists in the waste gas passing the wet desulfurization apparatus is also micron order and the collection performance of the sulfuric acid mists in the wet type electrostatic precipitator at a posterior stage can be enhanced.
However, the spraying of spray water for cooling the waste gas of Japanese Unexamined Patent Publication No. 2002-45643 is not easy in the adjustment of the particle diameters of sprayed water and water quantity for enlarging the particle diameters of the sulfuric acid mists. Further, the patent document 1 has a problem that a cooling means of spraying liquid for lowering the temperature of the waste gas must be provided between the dry type electrostatic precipitator and the wet desulfurization apparatus and facility cost is high.
Further, the stack gas desulfurization method of Japanese Unexamined Patent Publication No. 10-230128 is a method of condensing water vapor using fine dusts as nuclei, generating mists and improving the collection efficiency of the fine dusts, but the adjustment of the particle diameters of spraying water and water quantity is difficult in like manner as Japanese Unexamined Patent Publication No. 2002-45643 and the particle diameters of the sulfuric acid mists are not considered at all.